FIG. 1 shows a conventional Data Over Cable Service Interface Specification (DOC SIS) network. The cable modem (CM) 110 connects to the operator's hybrid fiber coax (HFC) network 140 and to a home network, bridging packets between them. Customer premise equipments (CPEs) 130 are connected to a cable modem 110. A CPE 130 may be embedded in the cable modem 110 in a single device or they may be separate. Examples of a CPE 130 include a gateway, a home router, a set-top box (STB), a personal computer, etc. The cable modem termination system (CMTS) 120 connects the operator's back office and core network to the HFC network 140. The CMTS 120 forwards packets between these two domains and between upstream and downstream channels on the HFC network.
The DOC SIS standards for the next generation HFC technology will be based on FDX. DOCSIS FDX will allow downstream and upstream transmissions to use the same radio frequency (RF) spectrum at the same time to increase the spectral efficiency in a cable network. As the downstream and upstream spectrums overlap in time and frequency in FDX, interference would occur between transmissions and receptions.
In order to implement FDX in a cable network, cable modems may be partitioned into interference groups. An interference group is a set of cable modems in which upstream and downstream signals of cable modems interfere with each other. Once interference groups have been identified the head-end (i.e. the CMTS 120) can schedule full duplex upstream and downstream packets to the cable modems so that they do not collide in time and frequency.
While a cable network operates in FDX, a cable modem in the FDX-mode cable network may operate on separate frequency bands for upstream and downstream (i.e. in a frequency division duplex (FDD) mode) to avoid upstream interference to downstream in an FDX band. A cable modem generally does not have dedicated filters or duplexers to protect a downstream channel from the high power upstream transmissions due to the very high cost of such devices and network capacity loss due to guard bands that filters would require.
Although a cable modem operates in FDD, upstream transmit power levels are significantly higher compared to the downstream received power levels. Part of the upstream signals leaks into the downstream data path with the level of isolations available with current components (directional couplers). In addition to that, a cable modem also gets reflections of the upstream signals from the HFC path coming back to the cable modem with considerable power levels.
The upstream self-interference in a cable network has two elements: adjacent channel interference (ACI) due to an in-band high power upstream signal, and adjacent leakage interference (ALI) due to out-of-band leakage of an upstream signal, which may be in-band of a downstream channel. ACI can cause saturation of a downstream analog-to-digital converter (ADC), which would require additional back off and hence losing the ADC dynamic range. On the other hand, ALI directly affects the modulation error ratio (MER) of a downstream channel acting as an in-band interferer. It may be needed to cancel the ALI and the ACI by around 30 dB in order to remove the effects of self-interference on a cable modem. The cable modem needs to deal with high power adjacent channels as well as the leakage from these adjacent channels into the downstream channels.